Circadian rhythms, roughly 24-hour rhythms in physiology and behavior, are widespread in nature and can be found in many plants and animals. Disruption of circadian rhythms has been linked to human health disorders ranging from jet lag to sleep and mood disorders and even to cancer. In eukaryotes, the circadian clocks that drive these rhythms are composed of networks of interlocked transcriptional feedback loops, in which positive factors induce the expression of negative factors that in turn repress expression of the positive factors. Although individual clock components are not conserved across kingdoms, clear analogies can be drawn between the organization of the circadian system and the `wiring'of the central clock, or oscillator, between disparate organisms. A fundamental understanding of how the circadian clock works in a variety of model systems will lead to new insights into the functioning of the human circadian system and its role in human health and disease. The long-term goal of the proposed research is to better understand the molecular basis of circadian rhythms in eukaryotes. These studies will be performed in Arabidopsis thaliana, a model organism with extensive genetic and genomic resources that is well-suited to circadian research. Some predicted components of the Arabidopsis central clock have not yet been identified, the biochemical functions of many known clock genes have not been determined, and it is not understood how the clock regulates growth and development. The proposed studies will use genetic, genomic, and biochemical techniques to address these fundamental questions. First, the molecular function of a clock-associated protein that is highly conserved across eukaryotes will be determined by a combination of genetic and biochemical studies in Arabidopsis and fission yeast. Second, a gene that acts close to the central clock will be cloned and characterized. Finally, the targets of a clock-regulated transcription factor that modulates the central clock and regulates organ size will be identified. These studies will yield important insights into the workings of the circadian clock and how it regulates growth and development in a complex eukaryote, information that ultimately may be used to improve human health through treatment of circadian disorders. PUBLIC HEALTH RELEVANCE: Almost all organisms possess an internal clock that generates roughly 24-hour rhythms in physiology or behavior. Disruption of this circadian clock in humans has serious negative consequences, causing sleep and mood disorders and perhaps even contributing to diseases such as cancer. To better understand the molecular basis of circadian rhythms, we are carrying out extensive genetic, biochemical, and genomic studies on the model organism Arabidopsis thaliana.